No criminal charges have been laid against three-time Nascar champion Tony Stewart after he struck and killed a rival driver during a race in upstate New York in the US.

Authorities questioned Stewart after the Watkins Glen International race, and Ontario County Sheriff Philip Povero said the driver was ''visibly shaken''.

Povero said Stewart had been cooperative in the investigation and no criminal charges were laid. Authorities were asking spectators and others to turn over any video they recorded of the crash.

A witness said it appeared Ward was trying to confront Stewart, the three-time Nascar Sprint Cup Champion. The video showed Ward standing to the right of Stewart's familiar No 14 car, which seemed to kick out from the rear and hit him.

Moments earlier, Ward and Stewart were racing side-by-side for position as they exited a turn.

Ward was on the outside when Stewart, on the bottom, seemed to slide toward Ward's car and crowd him toward the wall. The rear tyre of Stewart's car appeared to clip the front tyre of Ward's car, and Ward spun into the fence.

Povero said Ward, who was wearing a black firesuit and black helmet, had walked into the racing area and one car swerved to avoid him before he was struck by Stewart.

''The next thing I could see, I didn't see (the other driver) anymore,'' witness Michael Messerly said.

''It just seemed like he was suddenly gone.''

A spokesman for Stewart's racing team called Ward's death a ''tragic accident''.

The dirt track, about 50 kilometres southeast of Rochester, cancelled the remainder of the race and later posted a message on its Facebook page encouraging fans to ''pray for the entire racing community of fans, drivers, and families''.

Ward's website said he began racing go-karts in 1998 at age 4, but didn't start driving sprint cars until 2010.

The 20-year-old from Port Leyden, New York, was Empire Super Sprint rookie of the year in 2012 and this year was his fifth season racing the Empire Super Sprints.

SPRINT CAR HOBBY

Stewart often competed in extracurricular events like the race on Saturday (NZT Sunday). The multimillionaire was known to participate in races with purses worth less than US$3000 (NZ$3545) and drive alongside drivers of varying ages and talent levels.

The crash at the weekend came almost exactly a year after Stewart suffered a compound fracture to his right leg in a sprint car race in Iowa.

The injury cost him the second half of the Nascar season. Stewart only returned to sprint track racing last month, and won in his return, at Tri-City Motor Speedway in Michigan.

But the broken leg cost him the entire second-half of last season and sidelined him during Nascar's important Chase for the Sprint Cup championship.

Stewart wasn't cleared to get back in a race car until February, the day the track opened for preparations for Nascar's season-opening Daytona 500 began.

''Everybody has hobbies. Everybody has stuff they like to do when they have downtime, and that's just what it is for me,'' he said last month following his return to sprint car racing.

''That's what I like to do when I have extra time.''

Among Stewart's many business interests is his ownership of Ohio dirt track Eldora Speedway, which last month hosted the Nascar Truck Series, and his stake in Stewart-Haas Racing, which fielded cars for Stewart, Kevin Harvick, Kurt Busch and Danica Patrick.

He's struggled a bit this year since returning from his leg injury, and headed into today's race winless on the season and ranked 19th in the standings.

Stewart had been scheduled to start 13th today at Watkins Glen, one of just five remaining races for Stewart to either score a win or move inside the top 16 in points to grab a valuable spot in Nascar's Chase.

The site of Saturday night's crash was the same track where Stewart was involved in a July 2013 accident that seriously injured a 19-year-old driver.

He later took responsibility for his car making contact with another and triggering the 15-car accident that left Alysha Ruggles with a compression fracture in her back.

STEWART PULLS OUT

Tony Stewart has pulled out of the Sunday (NZT Monday) Nascar race at Watkins Glen.

Greg Zipadelli, competition director for Stewart-Haas Racing, said at a news conference that Stewart ''feels strongly'' about not racing following Kevin Ward's fatal accident.

The decision was an about-face for the organisation, which had said when the track opened that Stewart would be behind the wheel of his No 14 Chevrolet when the green flag waved.

''We gave Tony some time to sleep on it. He feels strongly this is the right thing to do,'' Zipadelli said.

''All you can do is what you feel is right, and we feel this is right. We get through today and do it the best we can as a group.